1. Technical Field
The present subject matter relates to motion detection. More specifically it relates to multi-output infrared radiation detectors and motion sensors using such an infrared detector.
2. Description of Related Art
Motion Sensors utilizing infrared (IR) radiation detectors are well known. Such sensors are often used in security systems or lighting systems to detect movement in a monitored space. An infrared detector detects changes in mid-infrared (IR) radiation having a wavelength of about 6-14 microns. These changes are due to temperature differences between a warm object, such as a warm blooded animal, and its background environment as the warm object moves through that environment. Upon detection of motion, motion sensors typically activate an audible alarm such as a siren, turn on a light, and/or transmit an indication that motion has been detected.
A typical infrared detector utilizes a pyroelectric or piezoelectric substrate with a detector element that consists of conductive areas on opposite sides of the substrate, acting as a capacitor. As the substrate changes temperature, charge is added or subtracted to the capacitor, changing the voltage across the capacitor. The amount of mid-IR radiation that hits the detector element determines the temperature of that area of the substrate, and therefore, the voltage across the capacitor that makes up the detector element. Some motion sensors utilize an infrared detector that includes multiple detector elements. To reduce the chance of false alarms, some infrared detectors include a pair of equally sized detector elements of opposing polarities. Non-focused out-of-band radiation, as well as ambient temperature changes or physical shock, is equally incident on both detector elements, thus causing the signals from the equal and opposite elements to roughly cancel one another.
Many motion sensors incorporate an optical array (comprised of optical elements, such as lenses, focusing mirrors, and so on) to be able to monitor a large space with a single infrared detector. The optical array directs the IR radiation from multiple monitored volumes onto the infrared detector, which sometimes includes filters to minimize the radiation outside of the desired mid-infrared range from reaching the infrared detector. Each of the monitored volumes is typically a pyramidal shaped volume extending into the space to be monitored with the apex of the pyramid at the motion sensor. Concentrations of radiation from each of the pyramids are projected by the optical arrays on to the infrared detector where they are superimposed, and different regions of the infrared detector are heated based on the amount of IR radiation received from the superimposed images. The detector elements on the infrared detector react to the localized heating by changing their voltage. The resultant change in voltage across the detector elements is monitored and used to detect motion in the space being monitored.